All pools have good days and bad days. Some do seem to have more bad days than most, but that is definitely not the case for Yiimp! My personal experience on Yiimp, is that it performs much better than most other pools that support the same coins.

Deposit stats are the worst place to look for your recent mining performance though. You should look at the pool stats instead.
What if the pool had some downtime? What if you had some internet issues? What if your rig has a couple of crashed cards? Note, I'm not saying that one of these was the case for the past 24h, just that looking at exchange deposits is really the worst way to go about measuring performance...

Cheers!